nope The latest Mac OSX update, released yesterday for Snow Leopard implemented changes that led Valve to see, "depending on the game, video settings and the hardware.. measured frame rate improvements from 15% to 120%."
Valve does warn, however, that the performance improvements are being seen "on iMac (Late 2009 and Mid 2010), Mac mini (Early 2009 and Mid 2010), Mac Pro (Early 2009), MacBook (Early 2009 and Mid 2010) and MacBook Pro (15-inch, Mid 2010) and MacBook Pro (17-inch, Mid 2010) models," while older systems are "already operating at the limits of the hardware."
A full (technical) explanation of the types of benefits Valve is seeing can be found on the Steam website. It includes fun buzzwords like "occlusion queries" and "floating point validation."
That same OSX update also included a fix for Blizzard's StarCraft 2 to remedy some unexpected crashing. Either way, Mac gamers will want to grab the latest OSX update if they are running Snow Leopard. If not, go get Snow Leopard first!